Sheet feeding unit

ABSTRACT

A sheet feeding unit including a feeding system, a controlling system to control the feeding system, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position, is provided. The controlling system controls the feeding system to carry the recording sheet at a second feeding speed, at a third feeding speed, and at a first feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus capable offorming an image on a recording sheet in an electrophotographic method,and particularly to a sheet feeding unit of the image forming apparatuscapable of controlling feeding speed of a recording sheet.

Conventionally, an image forming apparatus employing anelectrophotographic technology, such as a copier and a laser beamprinter, is known. Such an image forming apparatus is adapted to form alatent image corresponding to image data on a surface of aphotoconductive drum by laser beam or the like. When toner is adhered tothe latent image, a toner image is formed on the surface of thephotoconductive drum. The toner image is then in a developing unittransferred to a surface of a recording sheet that has been introducedvia a sheet feed roller of a sheet feeding unit, and is permanentlyfixed on the recording sheet in a fixing unit. This type of imageforming apparatus is often equipped with a feeding unit including feedrollers and a sensor to detect the recording sheet, and the recordingsheet is carried from a sheet inserting portion to the developing unitvia the feed roller, and the sensor.

The feed rollers are generally rotated by a stepping motor, and with therotating force carry the recording sheet nipped therebetween. The sensordetects a predetermined position of the recording sheet being carried,so that the image forming apparatus can synchronize emission of thelaser beam on the photoconductive drum based on the detected position ofthe recording sheet.

In Japanese Patent Provisional Publication No. HEI11-265096, a scanningdevice with an original sheet feeding unit to scan an image on anoriginal whilst the original is being carried to the scanning unit isdisclosed. In the scanning device, the original being carried is pausedwhen a front end of the original reaches a predetermined position at anupstream side of the scanning unit. Thereafter, according to timing ofthe scanning unit to scan the original, a pulse signal to acceleraterotation of the stepping motor, starting from a pull-in torque range, ina substantially low rate to avoid stepping out of the motor is inputtedto the stepping motor, so that the stepping motor can be rotated at aspeed that is beyond the pull-in torque range. In this configuration,the original can be carried to the scanning unit, which corresponds tothe developing unit of the image forming apparatus, at a feeding speedcorresponding to the scanning speed that may otherwise be limited withinthe pull-in torque range of the stepping motor. Thus, a faster feedingspeed of the original is attained.

A faster speed for feeding the recording sheet in the image formingapparatus as described above is preferable as well. The mechanism of theoriginal sheet feeding unit of the scanning device in theabove-referenced publication, for example, may be employed in the imageforming apparatus so that the recording sheet can be carried to thedeveloping unit at a faster speed without the limitation of the pull-intorque range. In the image forming apparatus, however, a feeding speedat the sensor and a feeding speed at the developing unit are required tobe equivalent, as detection of the predetermined position of therecording sheet and the emission of the laser beam to thephotoconductive drum must be synchronized. When the recording sheet iscarried at a faster feeding speed that requires torque beyond thepull-in torque range, the stepping motor requires to be controlled toaccelerate in a substantially moderate rate to avoid stepping out. Inorder for the motor to be properly accelerated, a substantial length tocarry the recording sheet is required before the recording sheet iscarried at the faster speed. Consequently, the image forming apparatusrequires substantial space corresponding to the length to carry therecording sheet between the feeding rollers and the sensor, and thus asize of the image forming apparatus may be increased.

SUMMARY OF THE INVENTION

In view of the foregoing shortcomings, the present invention isadvantageous in that a sheet feeding unit capable of feeding therecording sheet at a faster speed is provided, whilst an entireconfiguration of the image forming apparatus can be downsized.

According to an aspect of the invention, there is provided a sheetfeeding unit, including a feeding system, which is adapted to carry arecording sheet to a target position on a feeding path of the recordingsheet, a controlling system, which is adapted to control the feedingsystem so that the recording sheet is carried at a first feeding speed,at a second feeding speed being slower than the first feeding speed, andat a third feeding speed being faster than the first feeding speed, anda detecting system, which is provided in between the target position andthe feeding system and is adapted to detect a predetermined position ofthe recording sheet at a detecting position, is provided. Thecontrolling system controls the feeding system to carry the recordingsheet at the second feeding speed until the detecting system detects thepredetermined position of the recording sheet, at the third feedingspeed changed from the second feeding speed, and at the first feedingspeed changed from the third feeding speed, so that a period in whichthe predetermined position of the recording sheet being carried at thefirst feeding speed changed from the third feeding speed is carried fromthe detecting position to the target position is equivalent to a periodin which the predetermined position of the recording sheet being carriedconstantly at the first feeding speed is carried from the detectingposition to the target position.

Optionally, the feeding system may be adapted to carry the recordingsheet by a force provided from a motor.

Optionally, the motor may be a stepping motor.

Optionally, the second feeding speed may be caused by torque within apull-in torque range of the stepping motor.

Optionally, the predetermined position of the recording sheet mayinclude a front end of the recording sheet.

Optionally, a reflectance of the predetermined position of the recordingsheet may be different from a reflectance of the other area of therecording sheet excluding the predetermined position.

Optionally, a transmittance of the predetermined position of therecording sheet may be different from a transmittance of the other areaof the recording sheet excluding the predetermined position.

Optionally, the recording sheet may be carried at the first feedingspeed when the recording sheet is at the target position on the feedingpath.

According to an aspect of the invention, there is provided an imageforming apparatus, including a sheet feeding unit, which is adapted tocarry a recording sheet and to guide the recording sheet in a targetposition on a feeding path in the image forming apparatus, and an imageforming unit, which is adapted to form an image on the recording sheetat the target position on the feeding path, is provided. The sheetfeeding unit includes a feeding system, which is adapted to carry therecording sheet to the target position on the feeding path of therecording sheet, a controlling system, which is adapted to control thefeeding system so that the recording sheet is carried at a first feedingspeed, at a second feeding speed being slower than the first feedingspeed, and at a third feeding speed being faster than the first feedingspeed, and a detecting system, which is provided in between the targetposition and the feeding system and is adapted to detect a predeterminedposition of the recording sheet at a detecting position. The controllingsystem controls the feeding system to carry the recording sheet at thesecond feeding speed until the detecting system detects thepredetermined position of the recording sheet, at the third feedingspeed changed from the second feeding speed, and at the first feedingspeed changed from the third feeding speed, so that a period in whichthe predetermined position of the recording sheet being carried at thefirst feeding speed changed from the third feeding speed is carried fromthe detecting position to the target position is equivalent to a periodin which the predetermined position of the recording sheet being carriedconstantly at the first feeding speed is carried from the detectingposition to the target position.

According to an aspect of the invention, there is provided a sheetfeeding unit including a feed roller, which is adapted to carry arecording sheet to a predetermined location on a feeding path, astepping motor, which is adapted to rotate the feed roller, acontrolling system, which is adapted to control a feeding speed of therecording sheet being carried by the feed roller, a detecting system,which is adapted to detect a predetermined position of the recordingsheet, a retainer system, which is adapted to retain the recording sheetat an upstream side of the recording sheet from the feed roller withrespect to a normal feeding direction of the recording sheet, and a loadadjusting system, which is provided on the feeding path in between thefeed roller and the retainer system so that a load applied on the feedroller to carry the recording sheet is adjusted, is provided. Thecontrolling system is adapted to switch the feeding speed of therecording sheet from a first feeding speed to a second feeding speedbefore the predetermined position of the recording sheet is detected bythe detecting system. The first feeding speed corresponds to a rotationfrequency that is within a pull-in torque range of the stepping motor.The second feeding speed is faster than the first feeding speed andcorresponds to a rotation speed that is out of the pull-in torque rangeof the stepping motor. The load adjusting system is adapted to reducethe load applied on the feed roller to carry the recording sheet whenthe feeding speed of the recording sheet is switched from the firstfeeding speed to the second feeding speed.

Optionally, a part of the feeding path between the feed roller and theretainer system may be shortened so that the load applied on the feedroller to carry the recording sheet is reduced.

Optionally, the load adjusting system may be provided with a guideroller to guide the recording sheet in the feeding path. The guideroller may be adapted to be shifted so that the part of the feeding pathbetween the feed roller and the retainer system is shortened when thefeeding speed of the recording sheet is switched from the first feedingspeed to the second feeding speed.

Optionally, the load adjusting system may include a supporting member,which rotatably supports the guide roller and swingably rotates about anaxis. The supporting member may be attracted in a direction so that thepart of the feeding path between the feed roller and the retainer systemis elongated by the attraction.

Optionally, the recording sheet may be in a continuous form, and theretainer system may retain the recording sheet by rolling the recordingsheet around the retainer system.

According to an aspect of the invention, there is provided an imageforming apparatus, including a sheet feeding unit, which is adapted tocarry a recording sheet and to guide the recording sheet to apredetermined location on a feeding path in the image forming apparatus,and an image forming unit, which is adapted to form an image on therecording sheet at the predetermined location on the feeding path, isprovided. The sheet feeding unit may include a feed roller, which isadapted to carry the recording sheet to the predetermined location onthe feeding path, a stepping motor, which is adapted to rotate the feedroller, a controlling system, which is adapted to control a feedingspeed of the recording sheet being carried by the feed roller, adetecting system, which is adapted to detect a predetermined position ofthe recording sheet, a retainer system, which is adapted to retain therecording sheet at an upstream side of the recording sheet from the feedroller with respect to a normal feeding direction of the recordingsheet, and a load adjusting system, which is provided on the feedingpath in between the feed roller and the retainer system so that a loadapplied on the feed roller to carry the recording sheet is adjusted, isprovided. The controlling system may be adapted to switch the feedingspeed of the recording sheet from a first feeding speed to a secondfeeding speed before the predetermined position of the recording sheetis detected by the detecting system. The first feeding speed correspondsto a rotation frequency that is within a pull-in torque range of thestepping motor. The second feeding speed is faster than the firstfeeding speed and corresponding to a rotation speed that is out of thepull-in torque range of the stepping motor. The load adjusting system isadapted to reduce the load applied on the feed roller to carry therecording sheet when the feeding speed of the recording sheet isswitched from the first feeding speed to the second feeding speed.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

FIG. 1 is a diagram to illustrate a general configuration of an imageforming apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a diagram to illustrate a feeding unit according to the firstembodiment of the present invention.

FIG. 3 is a flowchart to illustrate a controlling process of a feedingspeed of the recording sheet according to the first embodiment of thepresent invention.

FIG. 4 is a diagram to illustrate the feeding speed of the recordingsheet in the controlling process according to the first embodiment ofthe present invention.

FIG. 5 is a diagram to illustrate a general configuration of an imageforming apparatus according to a second embodiment of the presentinvention.

FIG. 6 is a flowchart to illustrate a controlling process of feedingspeed of the recording sheet according to the second embodiment of thepresent invention.

FIG. 7 is a diagram to illustrate a relation between timing and thefeeding speed of the recording sheet in the controlling processaccording to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to the accompanying drawings, an image forming apparatus witha feeding unit of a recording medium according to an embodiment of thepresent invention will be described in detail. FIG. 1 is a diagram toillustrate a general configuration of an image forming apparatus 100according to a first embodiment of the present invention. The imageforming apparatus 100, which is often used as an output device for acomputer, is adapted to form an image on a continuous form recordingsheet (hereinafter referred to as a recording sheet) 10 in anelectrophotographic method by exposing a surface of a photoconductivedrum 4 to a laser beam modulated according to information (image data)inputted by a user.

As shown in FIG. 1, the image forming apparatus 100 is provided with afeeding unit 50 including a core roll 1, around which the recordingsheet 10 is rolled, feed rollers 2 a, 2 b, and a fed sheet sensor 3 (thefeed rollers 2 a, 2 b and the fed sheet sensor 3 as a feeding unit). Theimage forming apparatus 100 is further provided with a developing unit(an image forming unit) 60 including a photoconductive drum 4, anintermediate transfer roller 5, a secondary transfer roller 6, and anoptical unit 9 a for outputting laser beam 9, and with a dischargeportion 70 including fixing rollers 7 a, 7 b, and a discharged sheetsensor 8. It should be noted that in the present embodiment therecording sheet 10 is a rolled recording paper, however, the recordingmedium may not necessarily be in a continuous form, but may be a sheetof cut paper, for example.

The recording sheet 10 is forwarded in the image forming apparatus 100from the feeding unit 50 to the discharge portion 70 via the developingunit 60. When image forming (i.e., printing) is completed, a front endportion of the recording sheet 10 on which the image is formed isdischarged from the image forming apparatus 100 and is cut off with acutter and the like automatically or manually. In this configuration, anewly created front end of the remaining recording sheet 10 ispositioned in the discharge portion 70. When a new printing operation isconducted with the front end of the recording sheet in the dischargeportion 70 (a normal print operation), a front end portion of therecording sheet 10 between the developing unit 60 and the dischargeportion 70 is not provided with a new image, thus the front end portionis wasted. In consideration of this drawback, the image formingapparatus 100 may be configured to rewind the recording sheet 10 so thatthe front end is brought back to the feeding unit 50 before a newprinting operation is started (a reverse-and-print operation).

The recording sheet 10 is rolled around a core roll 1 that is rotatablysupported by a predetermined supporting member (not shown). A front endportion of the recording sheet 10 is unwound and directed to the feedrollers 2 a, 2 b by a guide 1 a that extends in an axial direction ofthe core roll 1. The core roll 1 is adapted to rotate in a clockwisedirection in FIG. 1 (hereinafter referred to as a normal direction) whenthe feed rollers 2 a, 2 b rotate to forward the recording sheet 10 andthereby pull the same. When a reverse-and-print operation is conducted,the feed rollers 2 a, 2 b rotate in respective reverse directions torewind the recording sheet 10 after a previous printing operation iscompleted or before a new printing operation is started. The core roll 1includes a tensile member (not shown) that rotates the core roll 1 in acounterclockwise direction in FIG. 1 (hereinafter referred to as areverse direction) so that the recording sheet 10 is tensioned betweenthe feed rollers 2 a, 2 b and the core roll 1.

The feed roller 2 a is rotated by rotating force provided from a drivemotor (a stepping motor) M. When the recording sheet 10 is fed in adirection from the feeding unit 50 toward the discharge portion 70(i.e., a normal sheet feeding direction), the feed roller 2 a rotates ina normal feeding direction (i.e., the counterclockwise direction in FIG.1), and when the recording sheet 10 is reversed (i.e., in a reversesheet feeding direction), the feed roller 2 a rotates in a reversefeeding direction (i.e., the clockwise direction in FIG. 1). In areverse-and-print operation, the front end of the recording sheet 10,which is positioned in a vicinity to the discharge portion 70, isbrought back to a position between the feed roller 2 a and the fed sheetsensor 3. A number of pulses of the drive motor M to be activated forthe reverse-and-print operation may be fixed in advance so that a lengthof the recording sheet 10 to be reversed can be fixed. The feed roller 2b is adapted to be rotated according to the rotation of the feed roller2 a, in an opposite direction from the rotating direction of the feedroller 2 a. It should be noted that the feed roller 2 b is mutually abutat a predetermined nip pressure with the feed roller 2 a, so that therecording sheet 10 is effectively carried between the two feed rollers 2a, 2 b.

The fed sheet sensor 3 is adapted to detect a predetermined position ofthe recording sheet 10. The predetermined position may be, for example,a front end of the recording sheet 10, or may be a position of a markprovided on a surface of the recording sheet 10. When a label paper isused as the recording sheet 10, for example, the recording sheet 10includes a base sheet, on which labels are arranged, and the labels, onwhich printing is provided. In such a case, a front end of each label ora marked position on the base sheet may be the predetermined position.

When a predetermined position of the recording sheet 10 such as thefront end thereof passes by the fed sheet sensor 3, the fed sheet sensor3 detects the same, and a timing of the laser beam 9 to be emitted iscalculated based on the detected predetermined position of the recordingsheet 10. It should be noted that with this configuration the image canbe formed on a predetermined position with respect to the recordingsheet 10.

The fed sheet sensor 3 may be provided with, for example, a lightemitting element and a light receiving element. In an embodiment of thefed sheet sensor 3, the light emitting element and the light receivingelement are arranged to oppose to each other, so that the recordingsheet 10 is carried therebetween. As the front end of the recordingsheet 10 passes by the fed sheet sensor 3, the light receiving elementstops receiving light from the light emitting element, and thereby thefront end of the recording sheet 10 is detected. When the label paper asthe recording sheet 10 is used, and transmittances of the base sheet andthe labels are different, the fed sheet sensor 3 can detect the frontend of the label by a difference in the transmittances. Further, whenthe transmittances of the marked position on the recording sheet 10 andof the other area are different, the fed sheet sensor 3 can similarlydetect the marked position by the difference in the transmittances. Inanother embodiment of the fed sheet sensor 3, the light emitting elementand the light receiving element may be arranged in adjacent to eachother. In this configuration, when the front end of the recording sheet10 passes by the fed sheet sensor 3, the light emitted from the lightemitting element is reflected on the surface of the recording sheet 10and is received by the light receiving element so that the front end ofthe recording sheet 10 can be detected. When the label paper as therecording sheet 10 is used, and the reflectance of the base sheet onlyand the reflectance of the labels with the base sheet are different, thefront end of the label can be detected. Further, when the reflectance ofthe marked position on the recording sheet 10 and of the other area aredifferent, the fed sheet sensor 3 can similarly detect the markedposition by the difference in the reflectance.

The photoconductive drum 4 is adapted to rotate in the counterclockwisedirection in FIG. 1. The photoconductive drum 4 is applied voltage anduniformly charged to a predetermined level, which is approximately +700V, by a charger (not shown). The photoconductive drum 4 is thereafterrotated and exposed to the laser beam 9 that scans the surface of thephotoconductive drum 4 in parallel with a rotation axis of thephotoconductive drum 4 (i.e., a main scanning direction) according tothe image data, and a latent image is formed on the surface of thephotoconductive drum 4, as regions where the latent image is formedgains a lower potential, for example as low as +100 V, due to an effectof the laser beam 9.

The photoconductive drum 4 with the latent image on the surface isfurther rotated, and between the region excluding the latent image onthe photoconductive drum 4, of which the electric potential isapproximately +100 V, and a surface of a developing roller 11, of whichthe electric potential is approximately +500V, the toner remains closelystuck to the lower-potential region i.e. the surface of the developingroller 11, without being transferred to the region of which the electricpotential is approximately +700 V and where no latent image exists.Consequently, the region excluding the latent image is not developed. Bycontrast, between the latent image region on the surface of thephotoconductive drum 4 and the surface of the developing roller 11, thetoner performs electrophoresis toward the lower-potential region. Thatis, the toner adheres to the latent image region on the surface of thephotoconductive drum 4. That is how the latent image on thephotoconductive drum 4 is developed, to turn into a toner image.

To an intermediate transfer roller 5 that rotates in the clockwisedirection, a transfer bias of a reverse polarity to the toner isapplied, which is approximately −100 V, so that the toner imagedeveloped on the surface of the photoconductive drum 4 is transferred asa primary step to the intermediate transfer roller 5, at the interfacebetween the photoconductive drum 4 and the intermediate transfer roller5.

The intermediate transfer roller 5 and a secondary transfer roller 6 aredisposed so as to oppose to each other across the paper path of therecording sheet 10, and mutually abut at a predetermined nip pressure.The secondary transfer roller 6 rotates in the counterclockwisedirection, and is applied voltage of approximately −1 kV. The tonerimage transferred to the surface of the intermediate transfer roller 5is transferred to the recording sheet 10 being carried along the paperpath at the interface with the secondary transfer roller 6, by theeffect of a transfer electric field, the nip pressure and so on, andthus the image is formed on the recording sheet 10. It should be notedthat the feeding speed at the developing unit 60 is configured to befaster than the feeding speed of at the feeding unit 50 caused by thefeed rollers 2 a, 2 b.

The secondary transfer roller 6 can be retracted in a position indicatedin the dotted line in FIG. 1. When the secondary transfer roller 6 isretracted, the recording sheet 10 is set apart from the intermediatetransfer roller 5. With this configuration, when the recording sheet 10is rewound in the reverse-and-print operation, as the feed roller 2 arotates in the reverse feeding direction, the surface of theintermediate transfer roller can be prevented from being damaged byfriction that may otherwise be caused by the recording sheet 10.

The recording sheet 10 that has passed the developing unit 60 isforwarded to the fixing rollers 7 a, 7 b. The fixing roller 7 a isadapted to apply heat to the recording sheet 10, whilst the fixingroller 7 b is adapted to apply pressure toward the fixing roller 7 a.With these fixing rollers 7 a, 7 b, the toner image is fixed onto therecording sheet 10. It should be noted that the fixing roller 7 b can beretracted in a position indicated in a dashed line in FIG. 1 so that thesurface of the fixing roller 7 a may not be damaged by friction when therecording sheet 10 is rewound.

The discharged sheet sensor 8 is adapted to detect a predeterminedposition of the recording sheet 10 with the toner image fixed theretothat passes by the discharged sheet sensor 8 itself. With thisconfiguration, the image forming apparatus 100 can detect an erroneouscondition of the recording paper 10. For example, the image formingapparatus 100 can be configured to determine that paper jam has occurredat some point between the fed sheet sensor 3 and the discharged sheetsensor 8, when the recording sheet 10 is not detected by the dischargedsheet sensor 8 after a predetermined period of time has passed since thepredetermined position of the recording sheet 10 had passed the fedsheet sensor 3.

Referring to FIG. 2, a feeding unit according to the first embodiment ofthe present embodiment will be described. FIG. 2 is a diagram toillustrate the feeding unit 50 with the developing unit 60 according tothe first embodiment of the present invention. In the feeding unit 50,the recording sheet 10 is carried by the feed rollers 2 a, 2 b. The feedroller 2 a is adapted to be rotated by the stepping motor M (see FIG.1), and the feeding speed of the feeding unit 50 is controlled bycontrolling rotation speed of the stepping motor M.

A positional relation between the feeding unit 50 and the developingunit 60 will be described. In the present embodiment, a position inbetween the feed rollers 2 a, 2 b, whereat the recording sheet 10 isnipped, is represented as a feeding position A, whilst a positionwhereat the recording sheet 10 is detected by the fed sheet sensor 3 isrepresented as a detecting position C. A position whereat the tonerimage is transferred to the recording sheet 10 is represented as atransfer position D. A length between the feeding position A and thedetecting position C is represented as a length L, and a length betweenthe detecting position C and the transfer position D is represented as alength S. Further, a position on the surface of the photoconductive drum4 whereat the laser beam is emitted is represented as an emissionposition E. A length including a circumferential length in thecounterclockwise direction in FIG. 2 of the photoconductive drum 4 fromthe emission position E to a position whereat the photoconductive drum 4is in contact with the intermediate transfer roller 5 (a position F) anda circumferential length in the clockwise direction in FIG. 2 of theintermediate transfer roller 5 from the position F to the transferposition D is represented as a path length (drawn in a thick line) N. Ata ready position B, whereat the predetermined position of the recordingsheet 10 is located when the image forming apparatus 100 is in a readystate and before the image forming apparatus 100 starts an printingoperation.

In the present embodiment, the fed sheet sensor 3 is arranged at apredetermined position so that the length S is equivalent to the pathlength N, and the laser beam 9 is adapted to be emitted at the emissionposition E when the fed sheet sensor 3 detects the predeterminedposition of the recording sheet 10. Thus, a latent image formed byemitting the laser beam 9 at the emission position E is formed as aprinted image on the predetermined position of the recording sheet 10,which was detected at the detecting position C. It should be noted thatthe fed sheet sensor 3 may be arranged at a position wherein the lengthS is longer than the path length N. In this configuration, the laserbeam 9 is emitted when a predetermined period passed after thepredetermined position of the recording sheet 10 had been detected atthe detecting position C by the fed sheet sensor 3, so that the imagecan be formed on the predetermined position of the recording sheet 10.

FIG. 3 is a flowchart to illustrate a controlling process of the feedingspeed of the recording sheet 10 according to the first embodiment of thepresent invention. As the process starts and the recording sheet iscarried in the normal sheet feeding direction by the feed rollers 2 a, 2b, the predetermined position of the recording sheet 10 is carried fromthe ready position B to the detecting position C. The predeterminedposition of the recording sheet 10 is carried to the detecting positionC at a speed V₁, which will be described in detail hereinbelow. In S10,the process examines whether the predetermined position of the recordingsheet 10 is detected by the fed sheet sensor 3 at the detecting positionC. When the predetermined position of the recording sheet 10 is detected(S10: YES), the process proceeds to S20. In S20, a feeding speed of therecording sheet 10 is accelerated in a predetermined rate until thefeeding speed reaches V₂(V₂>V₁), which will be described hereinbelow.When the feeding speed reaches V₂, the feeding speed V₂ remains constantuntil a predetermined period elapses.

In S30, the process examines whether the recording sheet 10 has beencarried for the predetermined period at the feeding speed V₂. The periodto carry the recording sheet 10 is determined based on a calculation,which will be described in detail hereinbelow, so that dimensions of ashaded area a in FIG. 4 are equivalent to dimensions of a shaded area b.When the predetermined period elapses (S30: YES), the feeding speed ofthe recording sheet 10 is decelerated in a predetermined rate until thefeeding speed reaches V₀ (V₁<V₀<V₂), which will be described in detailhereinbelow. When the feeding speed reaches V₀, the feeding speed V₀remains constant so that the predetermined position of the recordingsheet 10 is carried to the transfer position D at the feeding speed V₀(S40).

FIG. 4 is a diagram to illustrate the feeding speed of the recordingsheet 10 in the controlling process executed in the feeding unit 50according to the first embodiment of the present invention. When theimage forming apparatus is in the ready state (i.e., before and after aprinting operation,) the predetermined position of the recording sheet10 is located at the ready position B. In the diagram, when t as time is0, the predetermined position of the recording sheet 10 is detected atthe detecting position C (see FIG. 2). The feeding speed V₀, which isfor example approximately 200 mm per second, is equivalent to a feedingspeed of the recording sheet 10 at the developing unit 60 (i.e., thefeeding speed V₀ is equivalent to a speed of the image to proceed alongthe path length N). As an image forming operation is started, thestepping motor M (see FIG. 1) is activated to rotate the feed roller 2a. The feeding speed V₁ of the recording sheet 10 (i.e., a rotationspeed of the feed roller 2 a), which can be reached instantaneouslyafter the stepping motor M is activated, is determined based on arotation frequency within the pull-in torque range of the stepping motorM. In the present embodiment, the feeding speed V₁ (V₁<V₀) may be forexample approximately 100 mm per second. The recording sheet 10 is thuscarried at the feeding speed V₁ until the predetermined position of therecording sheet 10 reaches the detecting position C. A length betweenthe ready position B and the detecting position C is configured to besubstantially long so that the feeding speed of the recording sheet 10can be accelerated to V₁ within the length.

When the predetermined position of the recording sheet 10 being carriedat the feeding speed V₁ is detected (t₀), the stepping motor M isaccelerated in a predetermined acceleration rate. Meanwhile, the feedingspeed is increased from V₁ to V₀ when t is t₁, and from V₀ to V₂ when tis t₂ (V₂>V₀). From t₂ to t₃, the feeding speed is maintained constantat V₂. At t₃, the feeding speed is decelerated to V₀ in a predeterminedrate until t₄. Thereafter, the feeding speed is maintained constant atV₀, and the predetermined position of the recording sheet 10 reaches thetransfer position D at the feeding speed V₀.

A length wherein the predetermined position of the recording sheet 10 iscarried in a period between to (i.e., when the predetermined position isat the detecting position C) and t₄ is equivalent to a integration valueof the period from t₀ to t₄ and the feeding speed in the period (from t₀to t₄). In the present embodiment, the feeding speed is controlled sothat the dimensions of the shaded area a, which indicates a part of theintegration value, become equivalent to the dimensions of the shadedarea b, which indicates the other part of the integration value. Whenthe dimensions of the shaded area a and the shaded area b areequivalent, the length wherein the recording sheet 10 is carried in theperiod from t₀ to t₄ is equivalent to a length wherein the recordingsheet 10 is carried in a constant feeding speed V₀ in the same period.That is, when the feeding speed at the detecting position C (i.e., t₀)is V₁, which is slower than V₀, it is practically equivalent that therecording sheet 10 is carried at the constant feeding speed V₀ in thesame length. Therefore, emission of the laser beam 9 at the emissionposition E, which is based on the timing wherein the predeterminedposition of the recording sheet 10 is detected by the fed sheet sensor3, can be controlled based on the timing wherein the predeterminedposition of the recording sheet 10 is carried from the detectingposition C to the transfer position D at the constant feeding speed V₀.

As shown in FIG. 4, a period t₁ is required before the feeding speed V₁is accelerated to reach V₀. Therefore, in order for the recording sheet10 to be carried through the detecting position C at the feeding speedV₀, a substantial length that corresponds to the period t₁ is requiredbetween the ready position B and the detecting position C. However, whenthe recording sheet 10 is carried through the detecting position C atthe feeding speed V₁ with the configuration described above, a lengthbetween the ready position B and the detecting position C can beconfigured to be shorter than a corresponding length between the readyposition B and the detecting position C, wherein the recording sheet 10is carried through the detecting position C at the feeding speed V₀.Consequently, the length L, which is between the feeding position A andthe detecting position C can be configured to be shorter, and the feedrollers 2 a, 2 b can be arranged in positions closer to the fed sheetsensor 3.

In the present embodiment, as shown in FIG. 4, the feeding speed isaccelerated from V₁ to V₂ and is decelerated from V₂ to V₀ in constantrates, although the rates may not necessarily be constant. The feedingspeed in the period from t₀ to t₄ may be indicated in curved lines, aslong as the dimensions of the shaded area a and the dimensions of theshaded area b are equivalent.

According to the present invention, the length between the feed rollers2 a, 2 b and the fed sheet sensor 3 can be configured to be shorter, asthe recording sheet 10 can be detected by the fed sheet sensor 3 beforethe feeding speed of the stepping motor M reaches a desired speed.Therefore, the feeding unit 50 can be provided with more options indesign thereof and the image forming apparatus 100 can be downsized.

Next, a second embodiment according to the present invention will bedescribed hereinbelow.

FIG. 5 is a diagram to illustrate a general configuration of an imageforming apparatus 200 according to a second embodiment of the presentinvention. In the present embodiment, configurations corresponding tothe configuration of the previous embodiment is referred to by theidentical reference numerals, and description of those is omitted.

The recording sheet 10 is rolled around the core roll 1. The recordingsheet 10 is lead in between the feed rollers 2 a, 2 b, and a bufferingguide unit 300 is provided in a path of the recording sheet 10. Thebuffering guide unit 300 includes a guide roller 310, a pair ofsupporting members 320, and a pair of springs 340. The guide roller 310is a roller to guide the recording sheet 10, and is provided with thesupporting member 320 at each end. The guide roller 320 is configured tohave a length in a longitudinal direction thereof being substantiallyequivalent to or greater than a width of the recording sheet 10, whichis in parallel to the rotation axis thereof. Each of the supportingmembers 320 rotatably supports the guide roller 310 at one end, and isrotated about the other end. Each of the supporting members 320 isprovided with a spring 340, which applies attraction force to thesupporting members 340 respectively. Each supporting member 320 is anelongated plate member, which extends from a rotation axis thereof to arotation axes of the guide roller 310. At an approximately midpoint ofeach supporting member 320 in a longitudinal direction thereof, one endof the spring 340 is coupled to the supporting member 320. The other endof the spring 340 is fixed to a predetermined position (not shown) ofthe housing of the image forming apparatus 200. As shown in FIG. 5, thesprings 340 are adapted to attract the supporting members 320 from alower side of the image forming apparatus 200 (i.e., a lower side inFIG. 5) so that the supporting members 320 can be rotated in a clockwisedirection.

When the image forming apparatus 200 is in a ready state, i.e., when thestepping motor M is not activated, the buffering guide unit 300 is in aninitial position A. In this position, a downstream portion (i.e., afront end portion) of the recording sheet 10 with respect to the normalsheet feeding direction is held in between the feed rollers 2 a, 2 b,and an upstream portion, which is closer to a portion rolled around thecore roll 1, is tensioned toward an upstream end thereof by a rotationforce provided by an attraction member (not shown). The attractionmember is provided to the core roll 1, so that the rotation force of theattraction member in the counterclockwise direction is applied to thecore roll 1. From this initial position A, as the recording sheet 10 isfixed to the feed roller 2 a, 2 b at the downstream end thereof, and theupstream end is tensioned by the core roll 1, the tension accordinglytends to lift the guide roller 310 upwardly. That is, the guide roller310 is applied a rotation force to be rotated in the counterclockwisedirection by the recording sheet 10. It should be noted that, in theinitial position A, the rotation force to rotate the guide member 320 inthe counterclockwise direction applied by the recording sheet 10 and therotation force in the clockwise direction to rotate the guide member 340applied by the springs 340 are substantially equivalent. It should befurther noted that the springs 340 are adapted not to be expanded to anmaximum extent thereof when the springs 340 are in the initial positionA.

When a feeding operation is started, the stepping motor M startsrotating, and the feed roller 2 a is rotated accordingly to carry therecording sheet 10 in the normal sheet feeding direction. In order forthe recording sheet 10 to rotate the core roll 1 in accordance with therotation of the feed roller 2 a, torque against an impact load caused byinertia in the core roll 1 and recording sheet 10 is required. In thepresent embodiment, the guide roller 310 serves as a pulley until thecore roll 1 starts rotating, and the supporting members 320 are rotated(uplifted) in the counterclockwise direction in FIG. 5 against theattraction force of the springs 340. Simultaneously, the springs 340attract the supporting members 320, and thus the supporting members 340are swayed. As the supporting members 320 are rotated, the feeding pathof the recording sheet 10 becomes shorter, and the impact load on thestepping motor M is reduced. And as the supporting members 320 areswayed for a predetermined amount, the core roll 1 starts to rotate. Itshould be noted that the springs 340 are adapted not to be expanded tothe maximum extent thereof when the core roll 1 starts rotating.

It should be noted that, as the core roll 1 is rotated at a constantrotation speed, and the rotation speed of the feed roller 2 a is changedto be faster in an instant to carry the recording sheet 10 at a fasterspeed, the feed roller 2 a also requires torque against the impact load.When the feed roller 2 a is rotated at the faster speed, the supportingmembers 320 are swingably rotated in the counterclockwise direction.Accordingly, the impact load on the stepping motor M is similarlyreduced by the effect of the buffering guide unit 300. Thus, a length ofa path of the recording sheet 10 from the core roll 1 to the feed roller2 a is varied by the buffering guide unit 300, so that the impact loadis reduced. Further, it should be noted that the buffering guide unit300 prevents the recording sheet 10 from being loosened in the path sothat the recording sheet 10 can be properly tensioned as the supportingmembers 320 are swingably rotated. With this configuration, the steppingmotor M can be prevented from stepping out when the feeding speed of therecording sheet 10 (i.e., the rotation speed of the feed roller 2 a) isaccelerated instantly. That is, a maximum rotation frequency to bereached instantly with the stepping motor M maintained synchronized tothe pulse signals, can be increased.

FIG. 6 is a flowchart to illustrate a controlling process of feedingspeed of the recording sheet 10 according to the second embodiment ofthe present invention. The controlling process is executed before thepredetermined position of the recording sheet 10 is detected by the fedsheet sensor 3. It should be noted that feeding speeds V₀ and V₁referred to in the present embodiment are equivalent to the feedingspeeds V₀ and V₁ in the first embodiment. As the process starts, inS110, the stepping motor M is controlled to start rotating the feedingroller 2 a at a rotation speed corresponding to the feeding speed V₁ ofthe recording sheet 10. Thereafter, in S120, the feeding speed of therecording sheet 10 is controlled to be V₀, which corresponds to afeeding speed at the developing unit 60.

FIG. 7 is a diagram to illustrate a relation between timing and thefeeding speed of the recording sheet 10 in the controlling processaccording to the second embodiment of the present invention. In thediagram, when t as time is 0, the predetermined position of therecording sheet 10 is detected at the ready position B (see FIG. 2).

When the stepping motor M is activated as the feeding operation starts(t₀), the feeding speed of the recording sheet 10 is V₁. Thereafter att₁(t₁>0), the feeding speed is changed (switched) in an instant from V₁to V₀. At t₂ (t₂≧t₁), the predetermined position of the recording sheet10 transits the detecting position C (see FIG. 2). It should be notedthat t₂ may be concurrent with t₁. That is, the feeding speed V₁ may beswitched to V₀ at the detecting position C.

With the buffering guide unit 300 described above, the impact load uponswitching the feeding speeds of the recording sheet 10 is reduced by theeffect of the springs 340, and the stepping motor M can be preventedfrom stepping out, so that the feeding speed of the recording sheet 10can be instantly switched from V₁, which corresponds to a rotation speedwithin the pull-in torque range of the stepping motor M, to V₀, whichcorresponds to a rotation speed outside the pull-in torque range.Therefore, in the image forming apparatus 200, the length between theready position B and the detecting position C can be configured to beshorten, as the recording sheet 10 can be carried to the detectingposition C at the feeding speed V₀, which is switched from the feedingspeed V₁ in a shorter period after the stepping motor M is activated. Asthe length between the ready position B and the detecting position C canbe shorten, the feed rollers 2 a, 2 b can be arranged in closerpositions to the fed sheet sensor 3 accordingly.

Although examples of carrying out the invention have been described,those skilled in the art will appreciate that there are numerousvariations and permutations of the image forming apparatus and the drivedirection switching system that fall within the spirit and scope of theinvention as set forth in the appended claims. It is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or act described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

For example, the recording sheet 10 to be carried in the image formingapparatus 200 may not necessarily be in a continuous form, but may be acut sheet. When a cut sheet is used in the image forming appratus 200,the image forming apparatus 200 may be provided with a pair of rollersto hold the cut sheet at an upstream side of the feed rollers 2 a, 2 bwith respect to the normal sheet feeding direction so that the cut sheetcan be properly carried and/or the cut sheet may not be loosened. Insuch case, impact load on the stepping motor M by the pair of rollers issimilarly generated when the feeding speed of the feed roller 2 a isswitched to carry the cut sheet at the faster speed. Thus, the presentinvention can be also applied to an image forming apparatus that uses acut sheet of paper as a recording medium.

Further, it should be noted that a recoding sheet positioning member(not shown) is provided at the upstream side of the feed rollers 2 a, 2b with respect to the normal sheet feeding direction in the imageforming apparatuses 100, 200. The recording sheet positioning member isadapted to hold both sides of the recording sheet 10 so that therecording sheet 10 can be positioned properly in a width directionthereof. The recording sheet positioning member is linearly extendedalong the sheet feeding direction (i.e., in an approximately horizontaldirection in FIGS. 1 and 5). As the feed rollers 2 a, 2 b are located inpositions closer to the fed sheet sensor 3 (i.e., closer to a left-handside in FIGS. 1 and 5), as described above, the recording sheetpositioning member may also be located in a position closer to theleft-hand side in FIGS. 1 and 5. Thus, the image forming apparatuses100, 200 may be designed in a downsized configuration.

The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2005-137997, filed on May 11, 2005,which is expressly incorporated herein by reference in its entirety.

1. A sheet feeding unit, comprising; a feeding system, which is adaptedto carry a recording sheet to a target position on a feeding path of therecording sheet, a controlling system, which is adapted to control thefeeding system so that the recording sheet is carried at a first feedingspeed, at a second feeding speed being slower than the first feedingspeed, and at a third feeding speed being faster than the first feedingspeed, and a detecting system, which is provided in between the targetposition and the feeding system and is adapted to detect a predeterminedposition of the recording sheet at a detecting position, wherein thecontrolling system controls the feeding system to carry the recordingsheet at the second feeding speed until the detecting system detects thepredetermined position of the recording sheet, at the third feedingspeed changed from the second feeding speed, and at the first feedingspeed changed from the third feeding speed, so that a period in whichthe predetermined position of the recording sheet being carried at thefirst feeding speed changed from the third feeding speed is carried fromthe detecting position to the target position is equivalent to a periodin which the predetermined position of the recording sheet being carriedconstantly at the first feeding speed is carried from the detectingposition to the target position.
 2. The sheet feeding unit according toclaim 1, wherein the feeding system is adapted to carry the recordingsheet by a force provided from a motor.
 3. The sheet feeding unitaccording to claim 2, wherein the motor is a stepping motor.
 4. Thesheet feeding unit according to claim 3, wherein the second feedingspeed is caused by torque within a pull-in torque range of the steppingmotor.
 5. The sheet feeding unit according to claim 1, wherein thepredetermined position of the recording sheet includes a front end ofthe recording sheet.
 6. The sheet feeding unit according to claim 1,wherein a reflectance of the predetermined position of the recordingsheet is different from a reflectance of the other area of the recordingsheet excluding the predetermined position.
 7. The sheet feeding unitaccording to claim 1, wherein a transmittance of the predeterminedposition of the recording sheet is different from a transmittance of theother area of the recording sheet excluding the predetermined position.8. The sheet feeding unit according to claim 1, wherein the recordingsheet is carried at the first feeding speed when the recording sheet isat the target position on the feeding path.
 9. An image formingapparatus, comprising; a sheet feeding unit, which is adapted to carry arecording sheet and to guide the recording sheet in a target position ona feeding path in the image forming apparatus, and an image formingunit, which is adapted to form an image on the recording sheet at thetarget position on the feeding path, wherein the sheet feeding unitincludes a feeding system, which is adapted to carry the recording sheetto the target position on the feeding path of the recording sheet, acontrolling system, which is adapted to control the feeding system sothat the recording sheet is carried at a first feeding speed, at asecond feeding speed being slower than the first feeding speed, and at athird feeding speed being faster than the first feeding speed, and adetecting system, which is provided in between the target position andthe feeding system and is adapted to detect a predetermined position ofthe recording sheet at a detecting position, wherein the controllingsystem controls the feeding system to carry the recording sheet at thesecond feeding speed until the detecting system detects thepredetermined position of the recording sheet, at the third feedingspeed changed from the second feeding speed, and at the first feedingspeed changed from the third feeding speed, so that a period in whichthe predetermined position of the recording sheet being carried at thefirst feeding speed changed from the third feeding speed is carried fromthe detecting position to the target position is equivalent to a periodin which the predetermined position of the recording sheet being carriedconstantly at the first feeding speed is carried from the detectingposition to the target position.
 10. A sheet feeding unit comprising; afeed roller, which is adapted to carry a recording sheet to apredetermined location on a feeding path, a stepping motor, which isadapted to rotate the feed roller, a controlling system, which isadapted to control a feeding speed of the recording sheet being carriedby the feed roller, a detecting system, which is adapted to detect apredetermined position of the recording sheet, a retainer system, whichis adapted to retain the recording sheet at an upstream side of therecording sheet from the feed roller with respect to a normal feedingdirection of the recording sheet, and a load adjusting system, which isprovided on the feeding path in between the feed roller and the retainersystem so that a load applied on the feed roller to carry the recordingsheet is adjusted, wherein the controlling system is adapted to switchthe feeding speed of the recording sheet from a first feeding speed to asecond feeding speed before the predetermined position of the recordingsheet is detected by the detecting system, the first feeding speedcorresponding to a rotation frequency that is within a pull-in torquerange of the stepping motor, the second feeding speed being faster thanthe first feeding speed and corresponding to a rotation speed that isout of the pull-in torque range of the stepping motor, and wherein theload adjusting system is adapted to reduce the load applied on the feedroller to carry the recording sheet when the feeding speed of therecording sheet is switched from the first feeding speed to the secondfeeding speed.
 11. The sheet feeding unit according to claim 10, whereina part of the feeding path between the feed roller and the retainersystem is shortened so that the load applied on the feed roller to carrythe recording sheet is reduced.
 12. The sheet feeding unit according toclaim 10, wherein the load adjusting system is provided with a guideroller to guide the recording sheet in the feeding path, and wherein theguide roller is adapted to be shifted so that the part of the feedingpath between the feed roller and the retainer system is shortened whenthe feeding speed of the recording sheet is switched from the firstfeeding speed to the second feeding speed.
 13. The sheet feeding unitaccording to claim 12, wherein the load adjusting system includes asupporting member, which rotatably supports the guide roller andswingably rotates about an axis, and wherein the supporting member isattracted in a direction so that the part of the feeding path betweenthe feed roller and the retainer system is elongated by the attraction.14. The sheet feeding unit according to claim 9, wherein the recordingsheet is in a continuous form, and wherein the retainer system retainsthe recording sheet by rolling the recording sheet around the retainersystem.
 15. An image forming apparatus, comprising; a sheet feedingunit, which is adapted to carry a recording sheet and to guide therecording sheet to a predetermined location on a feeding path in theimage forming apparatus, and an image forming unit, which is adapted toform an image on the recording sheet at the predetermined location onthe feeding path, wherein the sheet feeding unit includes a feed roller,which is adapted to carry the recording sheet to the predeterminedlocation on the feeding path, a stepping motor, which is adapted torotate the feed roller, a controlling system, which is adapted tocontrol a feeding speed of the recording sheet being carried by the feedroller, a detecting system, which is adapted to detect a predeterminedposition of the recording sheet, a retainer system, which is adapted toretain the recording sheet at an upstream side of the recording sheetfrom the feed roller with respect to a normal feeding direction of therecording sheet, and a load adjusting system, which is provided on thefeeding path in between the feed roller and the retainer system so thata load applied on the feed roller to carry the recording sheet isadjusted, wherein the controlling system is adapted to switch thefeeding speed of the recording sheet from a first feeding speed to asecond feeding speed before the predetermined position of the recordingsheet is detected by the detecting system, the first feeding speedcorresponding to a rotation frequency that is within a pull-in torquerange of the stepping motor, the second feeding speed being faster thanthe first feeding speed and corresponding to a rotation speed that isout of the pull-in torque range of the stepping motor, and wherein theload adjusting system is adapted to reduce the load applied on the feedroller to carry the recording sheet when the feeding speed of therecording sheet is switched from the first feeding speed to the secondfeeding speed.